For the production of a glass preform added with fluorine methods have been proposed comprising depositing glass soot particles to form a soot preform and then dehydrating and vitrifying the soot preform by keeping or inserting it in a furnace containing an atmosphere comprising fluorine.
The first method is in Japanese Patent Kokai Publication (unexamined) No. 67533/1980 and comprises heating a soot preform in an atmosphere containing a fluorine-containing compound at a temperature not higher than 1,000.degree. C. and then heating and vitrifying the soot preform in an atmosphere of an inert gas at a temperature not lower than 1,400.degree. C. by keeping or inserting the soot preform 2 in an furnace 31 as shown in FIG. 1. Example 3 of Japanese Patent Kokai Publication (unexamined) No. 60938/1985 describes a similar method comprising heating a soot preform in an atmosphere containing SF.sub.6 at 1,000.degree. C. and vitrifying it in an atmosphere not containing SF.sub.6 at 1,600.degree. C. In addition, Japanese Patent Kokai Publication (unexamined) No. 86045/1985 describes a method comprising heating a soot preform in an atmosphere comprising a fluorine-containing compound at a temperature not lower than 1,000.degree. C. and lower than 1,400.degree. C. and then vitrifying the soot preform in an atmosphere of an inert gas at a temperature not lower than 1,400.degree. C.
The second method comprises heating a soot preform in an atmosphere comprising a fluorine-containing compound and an inert gas at a temperature not lower than 1,400.degree. C. to produce a glass preform containing fluorine, in which the soot preform 2 is inserted in a zone furnace 41 equipped with a heater 40 as shown in FIG. 3. A method similar to the above method is described in Japanese Patent Kokai Publication (unexamined) No. 5035/1986 and comprises heating a soot preform in an atmosphere comprising a fluorine-containing compound at 1,650.degree. C. The reference disclosure that "since the dopant is absorbed in the soot preform during vitrification of the glass, the treatment becomes easier at a higher temperature and vitrification proceeds at a higher rate than at a lower temperature so that the production cost is reduced". Examples 1, 2 and 4 of Japanese Patent Kokai Publication (unexamined) No. 60938/1985 comprise heating a soot preform in an atmosphere comprising a fluorine-containing compound at 1,600.degree. C. Further, claims 1 and 7 of Japanese Patent Kokai Publication No. 86049/1985 describe a method for producing dense glass by diffusing fluorine at a temperature in a sintering temperature range.
As the result of the study of the two methods, the following has been found:
According to the first method, the soot preform is heated in an atmosphere comprising the fluorine-containing compound in an apparatus as shown in FIG. 1 at a temperature not higher than 1,000.degree. C., the porosity of the soot preform is reserved. Thereafter, the soot preform is vitrified in an atmosphere of the inert gas. The analysis of the distribution of the refractive index of the produced glass preform revealed that the refractive index at a peripheral portion of the preform is larger than that of the central portion as shown in FIG. 2 in which R stands for an outer diameter of the glass preform. This means that the added amount of fluorine in the peripheral portion is less than that in the central portion. The reason for this may be that, since at the end of the addition of fluorine at 1,000.degree. C., the preform is still porous, added fluorine is dissipated during subsequent heating in the inert gas atmosphere at a higher temperature.
According to the second method, the soot preform is heated in an atmosphere comprising the fluorine-containing compound and the inert gas at a temperature not lower than 1,400.degree. C. and then vitrified, the produced glass preform has a distribution of the refractive index shown in FIG. 4, from which it is understood that the added amount of fluorine is smaller in the central portion. The reason for this may be that, since the addition of fluorine and vitrification occur simultaneously, a period of time to add fluorine is not long so that a sufficient amount of the fluorine-containing compound does not reach the central portion. When the soot preform was inserted in the furnace at a rate less than a half of the practically employed rate, fluorine was homogeneously added to the central portion of the preform. However, the total treating time was extremely extended.